The invention relates to a bone screw that comprises a screw head and a threaded shank, wherein an axial bore into which a plurality of radial bores set at a distance from each other open, extends through the threaded shaft, and said axial bore is open at the screw head side end, the radial bores each also being open at the radially outer end.
A like bone screw is known from U.S. Pat. No. 5,192,282.
In the course of the surgical treatment of bone fractures of the human skeleton, for example, a bone screw is intended to attach a bone fixation plate to the fractured bone in order to immobilize the fractured bone, thus allowing it to grow together again. Usually, such bone screws and the bone fixation plate are made of medical grade stainless steel or titanium.
The bone fixation plate comprises several bores, through each of which one bone screw is screwed into the bone. After the bone screw has been screwed into the bone completely, the screw head is countersunk in the bone fixation plate and fixed as free from play as possible.
The bone screw comprises a threaded shank with a self-cutting thread that drives itself into the bone, thus ensuring the thrust of the screw into the bone.
After the bone screw has been screwed into the bone, the bone fixation plate must be fixed to the bone, as permanently and free from play as possible. This is also dependent on the support the bone screw itself finds in the bone into which it is screwed. Normally, the support of the screw in the bone is improving in the course of time by bone growing, thus causing bony tissue to form around the threaded shank of the bone screw and to adhere to same on all sides.
This, however, can only be observed in younger patients with a healthy bone structure. In elderly patients, bony growth and, thus, the formation of bony tissue is strongly reduced. In case of osteoporosis patients, the situation is even more problematic, since such patients are, in addition, suffering from a decrease in bony substance with an increased susceptibility to fractures.
In such critical cases, the bony tissue fails to give any permanent support to the bone screw. For that reason, the bone screw requires reliable fixation in the bone by being cemented in the bone with bone cement.
Usually, a like bone cement is made from polymethyl methacrylate or related compounds. Normally, the bone cement, when in the processing state, is available as paste or as high-viscosity fluid.
At present, the procedure of cementing a bone screw in the bone comprises smearing the threaded shank with bone cement and then screwing it into the bone. When the screw is screwed in, however, the major part of the cement is stripped from the threaded shank, thus being prevented from entering into the bone at all.
Predrilling the screw hole and filling the cement into the bone beforehand is likewise of disadvantage because the bone cement is pressed out of the drilled hole when the bone screw is screwed in. Moreover, predrilling of the screw hole is not desired in most of the cases.
The bone screw known from the aforementioned U.S. Pat. No. 5,192,282 comprises a screw head and a threaded shank. An axial bore that passes completely through the screw, thus being open at either end, extends through the threaded shank. A plurality of radial bores that are open at their radially outer end extend from the axial bore.
This known bone screw is intended to evacuate air and liquids from the bone into which it is screwed, by connecting a vacuum pump to the screw head by means of an appropriate device.
The vacuum is intended to cause the bone cement that has been injected into the bone at a different place to distribute and spread in the bony tissue in an improved manner.
Moreover, the bone screw is intended to put liquid drugs into the bone therethrough. Although it is, in principle, suitable, the bone screw is not intended to attach the bone fixation plate to the bone. Neither is the bone screw itself cemented in the bone; instead, the cement mentioned is used to cement an endoprosthesis that is implanted at a different place of the bone.